In flexible ducts, a coated fabric or film is secured to a helical wire. The coated fabric can be secured by adhesives, crimping or sewing. The ducts are typically used to exhaust gases and vapors from a confined working area, i.e. the holds of ships, below ground electrical conduits, etc. The ducts should be flexible, extendable, collapsible, gas impermeable, fire resistant, abrasion resistant, lightweight and have strength in the fill (longitudinal) direction of the duct.
The fabric customarily used for duct material is a woven or non-woven material which is saturated and/or coated. The fabrics that are particularly useful for this purpose are square woven, oxford or duck.
To achieve the aforementioned properties for a flexible duct, the primary emphasis has been on varying the coatings that are applied to the fabric; such as applying coatings on both sides, modifying the chemical/physical characteristics of the coatings, applying multiple coatings at differing viscosities, etc.
Of the aforementioned desirable properties, the present invention results in specific improvements in increasing the strength of the duct in the fill direction, reducing the weight of the duct and minimizing the pin holes (gas impermeable) in the duct material. The advantages are achieved by the use of a particular fabric weave.
Broadly, flat fill yarns are used in simple weaves with spun warp yarns. The flat fill yarns are much larger in denier than the warp yarns so fewer of them can be used yet strength is increased in the filled direction which is more critical and improves the finish duct durability. Although there are fewer larger denier yarns, because they lay flat they reduce the interstices in the fabric thereby both enabling less coating to cover more completely and reducing the number of pin holes. Further, the fill yarns are less expensive to process so the fabric cost is also reduced.
In a preferred embodiment of the invention, a duct fabric comprises a woven fabric having flat filament fill yarns and spun warp yarns woven in a 1/1 plain box weave.